We carry around a list of stylesheets that can carry two different types
of thing:
1. Internal stylesheets specific to a page type (only for settings)
2. Themes
In this change I move the link generation for settings.css into
settings(), so Context.style_files is reserved just for themes.
We had two places where we extracted a base theme name from a list of
StylePaths. I consolidated that code to be a method on StylePath.
I moved generation of link tags for stylesheets into the page.html
template. With that change, I made the template responsible for special
handling of light.css (making it the default theme) and of the other
themes (marking them disabled). That allowed getting rid of the
`disabled` field on StylePath.
